It took some time for the Church to recognize the primacy of the Roman patriarch. By the 2nd century, however, there were references to Rome's superiority in the writings of the Church Fathers. Rome had the largest congregation of Christians by the year 100. Other Bishops would write to the Roman Bishop when they had questions. Finally, Christ had given the Keys of the Kingdom to St. Peter, who was the first Bishop of Rome. If Peter had been the prime apostle, then his successor would naturally be the prime Bishop - the Pope, or "Father". This is known as Petrine succession.

Persecutions of Christians occurred until 311, when the Emperor Galerius issued the Edict of Toleration, outlawing the practice. In 313, the Emperor Constantine went further with the Edict of Milan, which formally legalized Christianity. Finally, in 381, the Emperor Theodosius made Christianity the only legal religion in the Roman Empire.

There are several reasons for Christianity's "triumph". "Mystery" religions were very popular at this point in the Roman Empire; cults of Cybele, Isis, and Mithras were "it", and such cults included elements in common with Christianity: resurrection of the dead, secretive ceremonies, and an initiation ritual in which the believer is united with a deity. Christianity overpowered these relgions, however, for six reasons:

1. Superior organization, which was attractive to the Emperors
2. Believable literature, with real and familiar locations grounded in time and space
3. Equality among Christians, whether man or woman, rich or poor, slave or free
4. Extreme charity - Christians provided the only places to care for the sick and the aged
5. The doctrine of the incarnation provided for a unique and caring God
6. The Emperors took the side of Christianity.

Although Constantine's conversion made Christianity legal and vogue, many converts were not sincere. Some truly spiritual people, frustrated at the decline in commitment, separated themselves from society in order to focus more intently on God. The first to draw major interest was St. Anthony, an Egyptian hermit. People admired him, and wanted to copy him, so that he formed a "community of hermits" in the early 4th century.

Monasticism grew out of this desire to escape the influence of society. In 315, Pachomias instituted the first community of cloisteredmonks, called cenobites. The "Pillar Saints", or Stylites, were hermits who lived on the tops of pillars to test their patience; one managed to live on top of a pillar for 30 years. The first true Rule of Monasticism was drawn up by St. Basil in 360, and it is still standard in Eastern Christianity. John Cassian and St. Jerome carried this Rule to the West.

St. Benedict founded the standard Rule in the West in 543. He proposed three primary vows: stability (live in the monastery), obedience (to the abbot), and conversion of life (poverty and chastity). Monasticism in the West underwent many changes after St. Benedict, some of which resulted in the spread of Christianity throughout Europe.

In Benedict's time, most monks were not priests. By 750, most monks were priests as well, and they performed daily Mass, left the field labor they had been accustomed to, and instead focused on manuscript copying. The monks of Europe are primarily responsible for preserving the Greek and Roman classics. Beginning in the early 7th century, the Popes commissioned monks to be missionaries, the most famous of which was St. Augustine of Canterbury, who first converted the population of the British Isles. The monks, especially the Irish monks, Christianized Europe. They were admired because they had a purpose beyond their own salvation: the salvation of others, for which they prayed daily.